1. Field of the Invention
The present invention relates to a method of and apparatus for calculating a present position of a movable body, and a method of and apparatus for correcting the calculated present position of the movable body in a so-called navigation system, which displays the present position of the movable body on the basis of the position measured by use of measurement electric waves from satellites dedicated for positional measurement, or the position measured in a self-sustained manner by use of self-sustained or built-in type sensors.
2. Description of the Related Art
There is a so-called navigation apparatus, which displays a map including a position where a movable body in various kinds such as an automobile, an airplane, a ship etc., is currently located, and further superimposes a position mark indicating a position of the movable body at the currently located position on the displayed map, so as to perform a route guidance or navigation to the destination on the basis of the display. Among the navigation apparatuses, as on-vehicle navigation apparatuses which are mounted on vehicles or automobiles, there is a self-sustained or built-in type navigation apparatus and a GPS (Global Positioning System) type navigation apparatus as rough categories.
The former is a navigation apparatus, which obtains a moving direction and a moving distance of the movable body by use of self-sustained type or built-in type sensors such as a speed sensor, an angular speed sensor etc. equipped in or built in the pertinent vehicle, sequentially adds them with respect to a standard position so as to calculate the present position, and displays the present position mark and the corresponding map on the display picture plane on the basis of the calculated present position.
On the other hand, the latter is a navigation apparatus, which receives measurement electric waves from a plurality of GPS satellites launched in the space by a GPS receiver, calculates the present position of the movable body by means of a 3-dimensional measuring method or a 2-dimensional measuring method on the basis of the electric wave reception results, and displays the present position mark and the corresponding map on the display picture plane on the basis of the calculated present position.
Further, there is an on-vehicle navigation apparatus of a so-called hybrid type provided with both functions of the above described self-sustained type and GPS type.
According to each of the above explained navigation apparatuses, since the user (e.g. the driver) can grasp the present position of the self vehicle and the map in the vicinity of the present position in association with each other, it is possible to reach a destination without losing his or her way even in an area where the user has never experienced.
However, in the on-vehicle navigation apparatus especially in the above mentioned hybrid type, there is a problem that an accurate determination of the present position and the display thereof are inhibited due to various factors as explained below. Each concrete problem will be explained hereinbelow in detail.
Initially, a first problem is explained.
In general, the measurement electric wave from a satellite dedicated for positional measurement is transmitted such that time data indicating time is superimposed thereon. A GPS receiver in the on-vehicle navigation system of hybrid type outputs time information indicating the time at receiving the measurement electric wave, on the basis of the superimposed time data, together with present position information indicating a present position measured on the basis of the received measurement electric waves. Therefore, a CPU used in the on-vehicle navigation system of hybrid type is adapted to calculate the present position by synthesizing the present position information accompanied with the outputted time information based on the measurement electric wave, and the present position information calculated by the self-sustained type sensors. At this time of synthesizing, in the above mentioned on-vehicle navigation system of hybrid type, the time axis for the present position information, which is accompanied by the time information outputted on the basis of the measurement electric waves, and the time axis for the present position information calculated by the self-sustained type sensors are not synchronized with each other. Thus, there is a first problem that, in case that it is attempted to correct the present position information, which is calculated on the basis of the self-sustained type sensors, by use of the present position information, which is calculated on the basis of the measurement electric waves, it is difficult to correct it effectively due to a possible shift between these time axes of these present position informations.
Namely, in the above mentioned positional measurement based on the measurement electric waves, it is practically impossible to receive the measurement electric waves regularly by a constant cycle due to factors such as the position of the pertinent vehicle on the earth, the number of the GPS satellites able to receive the measurement electric waves and so on. More concretely, even if the measurement calculation by use of the measurement electric waves is supposed to be performed once every second, the calculation cycle may be changed to 1.5 seconds, for example. Further, since it takes about 1 to 2 seconds to actually finish calculating the measurement result (i.e. the present position information) after receiving the measurement electric waves, the calculated measurement result actually indicates the position of the movable body which is about 1 to 2 seconds before the present time.
In contrast to this, the positional measurement of the present position based on the self-sustained type sensors is almost accurately performed once every second, and the time required for completing the measurement calculation is short enough to neglect it in practice.
Therefore, the time axis for the present position information, which is outputted on the basis of the measurement electric waves, and the time axis for the present position information calculated by the self-sustained type sensors are not inevitably synchronized with each other. Consequently, the correction or the like for the present position information based on the self-sustained type sensors by use of the present position information based on the measurement electric waves, cannot be effectively applied due to the shift of these time axes.
Nextly, a second problem is explained.
As explained above, although the self-sustained type sensors output the present position information almost accurately once every second, there may be such a case that all of the present position information is not regularly inputted to the CPU. Thus, particularly, the distance information in the present position information based on the self-sustained type sensors becomes inaccurate, which is the second problem.
This point is explained in more detail with reference to FIG. 6A. In case that all of the present position information is regularly inputted from the self-sustained type sensors to the CPU, the distance information is regularly and sequentially added as shown in the upper half of FIG. 6A, so that the accurate distance information can be obtained. However, if a second distance data D2 is dropped out (not measured or inputted), although the self vehicle actually advances for a distance D as EQU D=.DELTA.d1+.DELTA.d2+.DELTA.d3,
the self-vehicle seems to advance, on the calculation, for a distance
D' as EQU D=.DELTA.d1+.DELTA.d3.
Accordingly, a distance error is generated. As a case where this distance information is dropped out and is not taken into the CPU, for example, there are a case where the distance data itself is destroyed by a physical noise generated on a communication line from the self-sustained type sensors to the CPU, a case where the distance data cannot be taken into the CPU for a while since the CPU performs a process for other information having a higher priority such as traffic jam information from the VICS (Vehicle Information and Communication System), which has been just put into practice, and so on. Then, the distance error becomes large as the case where the distance data cannot be taken happens more frequently, so that the present position cannot be accurately measured or displayed.
Finally, a third problem is explained with reference to FIG. 6B.
In general, in the on-vehicle navigation system, even if the self vehicle is actually traveling on a road for example, there may be such a case that the calculated present position indicates a position which is not on the road due to the measurement error or the like. In this case, a so-called map matching process is performed to correct the display position of the present position onto a position on a road, which is the closest to the measured present position P.sub.3 obtained by calculating the moving vector V with respect to the previous display position of the present position as a standard, and displays the present position on the correct displayed position (i.e. a presently map matched position P.sub.2). In the above mentioned on-vehicle navigation system, after calculating the present position and then performing the map matching process, the position after the map matching process is displayed as the present position.
However, in general, it takes a long processing time period to perform this map matching process. Thus, as the movable body advances during this processing time period, although the self-vehicle is actually positioned at a true present position C, the position displayed as the present position is the presently map matched position P.sub.2. Consequently, the error is certainly included in the displayed present position by the amount corresponding to the distance between the true present position C and the presently map matched position P.sub.2. In this way, the present position cannot be accurately displayed.